PAGE 127 OF 150 · POWER SUPPLIES & BATTERIES
Linear Voltage Regulators
A linear regulator wastes extra voltage as heat.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
How they work
A linear regulator such as the 7805 holds its output at a fixed voltage by burning off the excess as heat. The LM317 allows an adjustable output.
Heat
Power lost is (input voltage − output voltage) × current. Taking 12 V down to 5 V at 200 mA wastes 7 × 0.2 = 1.4 W, enough to need a heat sink.
- Input must exceed output by the dropout voltage, about 2 V for a 7805.
- Add a small capacitor at input and output, commonly 0.33 and 0.1 microfarads.
- Low-dropout types work with a smaller gap.
Linear regulators are quiet and simple, but wasteful with large voltage gaps.
REMEMBER
The bigger the voltage gap, the more heat is wasted.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
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